1. Field of Disclosure
The following disclosure generally relates to disk drives and more particularly to a scheme for detecting defects in a hard drive assembly.
2. Description of Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is typically recorded on concentric tracks on either surface of one or more magnetic recording disks. To facilitate the storage and retrieval of data in an orderly manner, disks are typically organized in blocks called sectors. These sectors are located on the disk by a set of unique identifiers called cylinder (or track), head and sector number. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms. These actuator arms are maneuvered via voice coil motors, wherein the voice coil motor is excited with a current to rotate the actuator and move the heads.
The movement of the actuator is controlled by a servo system. One well known type of servo system is referred to as a dedicated servo, wherein one area of one of the disk is dedicated to servo information. By using this servo information, the actual radial positions of the heads can be determined, and after comparison with desired head position information, control signals can be sent to the actuator arm to adjust the head position accordingly.
The servo system typically sends control signals to the actuator in accordance with a position error signal (PES) derived from the servo information. Typically, the PES has a magnitude indicative of the relative distance between the head and the center of a track and a polarity indicative of the direction of the head with respect to the track center. Additionally, the PES generally is generated by the servo system by comparing the relative signal strengths of burst signals on the disk surface. It is common during read/write operations to compare the absolute value of each PES sample to a predetermined safe-threshold value in order to verify the correct positioning of the head. It is generally known in the art that there are two situations which may give rise to a PES value which exceeds a safe-threshold value. First, it is possible that the head is misaligned to such an extent that its distant from the center of the track is sufficient to exceed this safe-threshold value. Second, it is likewise possible that a defect exists in the servo information, resulting in a misreported PES value, despite the fact that the head is positioned correctly. This latter scenario is what has led to the practice of using high PES values as an indication that there may be a defect in the servo burst area.
As is generally known in the prior art, using a high PES value to detect servo defects is a cumbersome process. This is due to the fact that the sector with the highest PES value often is not the sector which actually contains the defect. Thus, it is generally considered necessary to check several of the sectors surrounding the high PES sector before labeling any given sector as defective.
In addition to the traditional method for detecting defects being cumbersome, relying on a high PES for detecting defects may also result in mapping continuous sectors as real servo defects. Similarly, large changes in PES may be caused by errors which occurred during the servo track writing process. Whereas the term ‘real servo defects’ is used herein to describe physical defects in the burst pattern areas of servo sectors, the term ‘closure servo defects’ will be used to describe those areas exhibiting a large change in PES created by a discontinuity of the written-in servo track. It should further be appreciated that such a discontinuity may create a shock during track following, often leading to overshooting. The result of this is that several sectors after the closure defect often exhibit poor track following characteristics.
In addition to mistaking closure servo defects for real servo defects, relying simply on a high PES for mapping defects may also result in mapping virtual servo defects as real servo defects. The term ‘virtual servo defect’ is used to describe the situation where a sector's PES value is too high to guarantee stable writing conditions. Although the sector may not exhibit any other symptoms of damage, its write gate is typically disabled since the high PES value creates doubt as to the stability of the sector.
Accordingly, there is a need for an improved approach to detecting servo defects on a disk in a disk drive assembly which can better differentiate between the different types of defects, is less cumbersome, and more accurate.